BAGHDAD, Iraq (CNN) -- Shiite cleric Muqtada al-Sadr threatened "open war," against the American "occupiers" and not the Iraqi government, according to a letter read by a top aide during Friday prayers.

Supporters of Muqtada al-Sadr hold up his photo before noon prayers in Baghdad's Sadr City on Friday.

"When we threatened to declare an open war until the liberation, we meant war against the occupier," al-Sadr said in his letter. "There is no war between us and our Iraqi brothers -- no matter what their nationality, race or sect. The blood of Iraqis are forbidden on you."

He encouraged not only his Mehdi Army militia to fight U.S. troops, but also every Iraqi, including the nation's security forces which are in league with U.S. troops.

"I direct my speech to all government security forces and others, and all the religious and political forces, if this open war is between us and the occupier, you should not interfere in the favor of the occupier," the letter said. "We want to liberate you and your lands from the occupiers, so our government would be a full sovereign government."

On Saturday, al-Sadr issued what he called a "last warning" and told his followers in Sadr City to fight the "occupier" there.

Despite the threat, al-Sadr has a freeze on Mehdi Army military activities, said Saleh al-Ageili, a spokesman for Sadr's parliamentary bloc. Al-Sadr said earlier this month that if he called off the freeze, he would issue a separate statement, al-Ageili said.

Al-Sadr praised members of his Mehdi Army for obeying the freeze in his letter Friday.

"The enemy is waiting for the chance to attack you," al-Sadr said. "Damn the occupier and his work, which raised the hand of violence and death against your children, women, elders and your scholars."

He continued, "And you -- still patient -- you showed the clear, white face of al-Sadr and his family, and you have been the people of peace, the people of Islam."

Al-Sadr said he would not accept the long-term agreement which the U.S. and Iraqi governments are crafting to replace the current U.N. mandate for multi-national forces in Iraq. The U.N. mandate expires in December.

"There should be no long-term security agreement between the government and occupier, no matter how hard they fight us and no matter how much of our blood is spilled," he said. "We will not accept to divide Iraq and to steal its wealth."

Al-Sadr's supporters have accused Prime Minister Nuri al-Maliki's government of not holding up its end of the cease-fire agreement, which includes freeing non-convicted prisoners from the Sadrist movement and halting what they call "illegal" raids on the cleric's followers.

Al-Sadr's message came after at least 11 people were killed and 36 others were wounded in overnight battles in northern Baghdad's Sadr City neighborhood, an Iraqi Interior Ministry official said.

U.S.-backed Iraqi forces and fighters loyal to al-Sadr have been fighting for several weeks in the sprawling Shiite slum.

The U.S. military launched overnight airstrikes on Sadr City, the official said.

Violence in Sadr City reduced somewhat after al-Sadr called on his supporters to halt attacks on March 30, but it's still heavy, the U.S. military said.

Other developments

• Tariq Aziz, one of the best-known faces of Saddam Hussein's former regime in Iraq, will go on trial Tuesday with six other defendants for their alleged roles in the execution of dozens of Iraqi merchants in 1992. Iraqi prosecutor Jaafar al-Moussawi said the merchants, summoned to the Interior Ministry, were blamed for rising prices, given a one-day trial and sentenced to death. Aziz's lawyer, Badee Aref, says his client had nothing to do with the executions.

• A minivan carrying five armed men exploded Friday afternoon near an Iraqi army checkpoint in northern Baghdad, wounding three soldiers and killing the men in the car, an Interior Ministry official told CNN. The official said the men began firing guns after the soldiers gestured for the vehicle to stop. The minivan exploded when the soldiers fired back, the official said.

• Four masked gunmen shot and killed a journalist working for a Basra radio station run by a Shiite political party, a police official said. Jassim al-Batat, 38, was driving his car Friday when gunmen opened fire as they drove past him in Qurmat-Ali, just north of Basra, the official said. Since the war began in March 2003, 157 journalists and 54 media assistants have been killed in Iraq, Reporters Without Borders said.

• The U.S. military also said a U.S. soldier was killed by a roadside bomb south of Baghdad Thursday. The death brings to 4,051 the number of U.S. military personnel killed in the Iraq war since it began in March 2003. E-mail to a friend